I am sure there is someone else on the message board that can give a better answer but here is what I have done in the past in a similar situation.

Your ‘buckets’ do not exist by themselves. Your calculated field assigns a bucket to each row of data in your data table. So the problem is that you have no rows of data that have “2-7 Days”, thus it does not show up in your bar chart. It is not a count of zero, it doesn’t exist at all. So the only way to change this is to add a new row to your data table that will result in a “2-7 Days” value. Although not ideal, in the past I have created an excel spreadsheet with a single column of data. That column contains the bucket names in each row, the same names you have in your calculation (e.g. 0-1 Days, 2-7 Days,....). You then import that spreadsheet into your Tableau workbook and join it to your main table using the ‘bucket‘ field and a full outer join (Must be full). That will add one row to your table for “2-7 Days” but that row has no other data in it. You may have to modify your method of counting (e.g. countd() on an id field or something) otherwise the ’number of records‘ field will show this as 1 for a count when there really is no data you want counted.